Electromagnetic actuators are well known. In many applications, the output force of the actuator is controlled by and is a function of an electrical control or command signal, and as such can be used in a variety of applications. One type of electromagnetic actuator is the "linear" actuator wherein output force is linearly proportional to the input electrical current. For example, as described in U.S. Pat. No. 4,892,328, issued Jan. 9, 1990 and assigned to the assignee of the invention disclosed in the present application (hereinafter the "'328 patent"), one type of a linear electromagnetic actuator is employed as part of an electromagnetic strut assembly in a vehicle suspension system for controlling the level and orientation of the vehicle sprung mass relative to the vehicle unsprung mass. More particularly, a radially polarized permanent magnet is carried by the sprung mass and is disposed coaxially within a coil carried by the unsprung mass. A current is applied to the coil in an amplitude and polarity selected to develop an axial force which maintains the orientation of the sprung and unsprung mass in a predetermined orientation.
In the '328 patent, the radially polarized magnet is mounted on an elongated rod made of magnetically "soft" iron to provide a flux return path to the radially inner pole of the magnet. Accordingly, the magnetic flux which is developed from the radially outward pole and which radially passes through the coil must be directed by some means to either end of the elongated rod so that there is a complete magnetic circuit. However, the flux path external of the coil for return to the elongated rod cannot again pass through the coil. Otherwise, the coil current interacting with the return flux will develop a force in opposition to the force developed by the coil current interacting with the flux at the radially outer pole of the permanent magnet. Therefore, to keep the flux path between the ends of the elongated rod external of the coil, the coil is housed in a non-magnetic material housing with the flux path external of the coil being through the vehicle spring and upper and lower spring seats between which the elongated rod carrying the radially polarized magnet is connected. However, the non-magnetic material housing provides a high reluctance flux path which decreases the flux density across the coil from the outer radial pole of the permanent magnet, thereby decreasing the overall efficiency of the device.
An improved linear electromagnetic actuator, as described in the '343 patent, overcomes the above described limitation. The actuator of the '343 patent is capable of providing relatively large output forces in response to relatively small level command signal. As set forth in the '343 patent, the actuator comprises first and second cylindrical assemblies coaxially mounted and movable relative to one another along a common axis of relative movement.
The first assembly includes three coils, each of different radius, disposed coaxially about the axis of relative movement. The second assembly includes at least a pair of axially spaced apart cylindrical magnets, each radially polarized so that flux is directed in a radial direction from each magnet between the inner and intermediate coils. At least a second pair of similar magnets are positioned between the intermediate and outer coils. The magnets are polarized so that one of the magnets of each set provides flux in a radially inward direction while the other magnet of each set provides flux in a radially outward direction. The magnets providing the inwardly directed flux are axially aligned, as are the magnets providing the outwardly directed flux so that the magnets of each polarity orientation provide all of the radial flux through the same sections of the coil assembly.
The second assembly also includes a center core member positioned inside of the inner coil and a cylindrical tube positioned around the outer coil, both coaxial with the axis of relative movement to provide an axial return path for the radial flux developed by the magnets. Brushes are provided between the sets of magnets and at the opposite ends of the magnets for applying control current in the coils in one direction through the inwardly directed flux and in the other direction to the outwardly directed flux so that the current/flux force created in accordance with Lorenz' Law will be additive. In the embodiment shown in the '343 patent, the magnets, core element and outer cylinder all move relative to the coils in response to the force provided. The magnets are preferably made of a high magnetic energy product material producing relatively high flux density, such as for example, neodymium-iron-boron or samarium cobalt.
The actuator shown in the '343 patent provides relatively high output forces in response to relatively low command signals as compared to the electromagnetic device disclosed in the '328 patent. Since the first and second assemblies move relative to each other, either assembly may be used to actuate an external device. However, the coil assemblies alone, whether structurally stationary with respect to the actuator device or coupled to the actuator device, may not be sufficiently structurally rigid to withstand the forces applied between the two assemblies. Furthermore, if the core, outer cylinder and intermediate cylindrical sections which carry the permanent magnets are coupled to the actuated device, additional weight is added to the actuated mass, requiring higher currents or reducing bandwidth.
To impart sufficient structural integrity to the coil, the coil may be carried by the core, and the radially polarized magnets may then move in a separate assembly external of the coil as shown in either FIG. 4 or FIG. 6 of the '158 patent. In FIG. 4 of the '158 patent, electrical connection is made to the coil through brushes which are carried with the moving magnet assembly. In FIG. 6 of the '158 patent, brushes are eliminated by providing for a first half of the coil to be counterwound with respect to the second half of the coil. The magnets with the flux radial in the first direction will move along the first coil half and the magnets with the opposite radial polarization will move along the counterwound coil half such that a current through the coil will interact with the flux to develop an additive force so that the first and second assemblies move relative to each other.
Yet another type of actuator providing support for the coil is shown in Prior Art FIG. 1. More particularly, FIG. 1 shows shows a prior art actuator 10 which is commercially available from Northern Magnetics of Van Nuys, Calif. The prior art actuator 10 includes magnetic flux conductive cylindrical case 12 having an inner wall 14 extending between a first end 16 and second end 18 of the case 12. An electrical current conductive coil 20 is wound on a nonmagnetic material coil carrier 21. The carrier 21 is coaxially secured within the case 12 with the windings of the coil 20 being intermediate the carrier 21 and the inner wall 14. The windings are made from thin copper wire.
A core assembly 19 includes an axially polarized cylindrical magnet 22 having a first magnetic pole at its first end 24 and a second opposite magnetic pole at its second end 26. A first disc shaped magnetic flux conductive material pole piece 28 is attached to the first end 24 of the permanent magnet 22. A second magnetic flux conductive material pole piece 30 is connected to the second end 26 of the permanent magnet 22.
The permanent magnet 22 and the first and second pole pieces 28, 30 are coaxially mounted to a cylindrical rod 32 which, in turn, is coaxially received by end caps 34, 36 in axial slidable engagement. Each end cap 34, 36 is attached to the cylindrical case 12. The rod 32 is received in slidable engagement in coaxial bores 38, 40 in each respective end cap 34, 36. It is to be noted that the cylindrical rod 32 and end caps 34, 36 are of nonmagnetic material. The cylindrical bores 38, 40 may include bearings (not shown) to reduce frictional losses. The actuator 10 is one type of moving core actuator.
Accordingly, the first pole piece 28 provides flux in a radial first direction across the coil 20 and the second pole piece 30 provides flux in the opposite radial direction across the coil 2. Ideally, the flux is confined to the case 12 in the axial section between the present position of the first pole piece 28 and the second pole piece 30. Thus, if current is put into the coil 20 at its midpoint 42, with the current return being at a first end 44 and a second end 46, with each end 44, 46 connected in common, then the current flux cross product with each pole piece 28, 30 will be additive. Alternatively, the coil 20 of the prior art actuator 10 may also be counterwound at either side of the midpoint 42 as set forth in the '158 patent.
However, the ideal flux confinement does not exist. Since the magnet 22 is axially polarized, there will be leakage of the flux from the first pole piece 28 to the second pole piece 30 at the point they are attached to the rod 32 through the center bore of the rod 32. Furthermore, a flux path will emanate from the tops of the first pole piece 28 and the second pole piece 30 external of the case 12 since the tops of the pole pieces merely extend the axial polarization of the magnet 22. Accordingly, not all the available flux from the magnet 22 is being utilized to provide radial flux in confined axial regions of the coil 20. This flux loss reduces the total output power available from the actuator 10.
Furthermore, to obtain a radially polarized high flux density which remains constant in the axial direction, each pole piece 28, 30 must be relatively thin in their axial dimension. Otherwise, the flux density will be at a maximum where each pole piece 28, 30 is adjacent to cylindrical magnet 22 and decrease in the axial direction away from the cylindrical magnet 22. Enlarging the axial dimension of the pole pieces 28, 30 will also not change the total flux across the coil since the total available flux is determined by the permanent magnet 22. Therefore, only a small portion of the total current within the coil 20 is available to interact with a high flux density for producing an axial force since the high density radial flux is confined to a very narrow axial region. Therefore, much higher currents and power consumption are required for the prior art actuator 10 to achieve the same types of output forces available through the actuators disclosed in the above-referenced patents.
Another limitation on the total output force available from the prior art actuator 10 is due to the coil carrier 21 being disposed between the pole pieces 28, 30 and the coil 20. The nonmagnetic material carrier 21 enlarges the gap in which the flux is confined, thereby reducing field strength.